1. Field of the Invention
This invention relates to rechargeable battery power systems for space vehicles. More particularly, it relates to the reconditioning of batteries so as to minimize the reduction of battery effectiveness, due to the memory effect and unequal cell voltages.
2. Description of the Prior Art
Spacecraft normally carry an electrical power subsystem for providing the requisite electrical energy needed to operate the spacecraft subsystems such as: attitude control, communication equipment, sensors, cameras, radiometers, and other scientific instrumentation. Such spacecraft employ solar cells arranged in various types of arrays for converting direct solar energy into electrical energy.
The solar cell delivers power during the sunlight portions of the orbit, but since the subsystems will require power during the dark portions of the orbit or eclipse season, an auxiliary or secondary source of energy must be incorporated in the power subsystem. A storage or rechargeable battery is utilized conventionally as the secondary source.
A secondary battery may develop an undesirable condition that results in the reduction of its output voltage under a variety of circumstances. This battery condition, commonly called the "memory effect," results from repetitive charge-discharge cycles from a continuous trickle charging occurring for long periods of time or from operations at elevated temperatures. The memory effect, to be discussed more fully, hereafter, results in a lowering of cell voltage. Under load, the battery is unable to deliver its rated capacity. Although such circumstances usually occur in geo-stationary synchronous altitude orbits, the memory effect occurs in other types of orbits as well.
The undesired reduction of the battery's output voltage may be minimized, as known, by maintaining the battery both in a temperature controlled environment and by periodically performing a "reconditioning cycle." This reconditioning cycle, to be described more fully hereafter, discharges the stored energy from each cell of the battery and recharges each cell of the battery at a relatively high rate of charge.
One system for in-orbit reconditioning of nickel-cadmium batteries places a resistor across the entire battery and discharges the battery until its voltage is equal to approximately 1.0 volts per cell. Such a voltage discharge limitation is imposed to minimize the probability of cell voltage polarity reversal, a condition caused by mismatch in cell characteristics that are present initially or develop with time. Such reversal can cause an electrical failure or the catastrophic rupture of a cell or group of cells. This cell voltage discharge limitation leads to incomplete discharge of cell stored energy and does not correct unequalized cell voltages thereby reducing the effectiveness of the reconditioning cycle.
Another system for reconditioning batteries is described in U.S. Pat. No. 3,454,854 issued on July, 8, 1969 to F. E. Ford et al. and assigned to the Secretary of the Navy. According to this system when battery voltage falls below a selected level, the battery is removed from the load and the cells are reconditioned by first discharging through a transistor placed across the battery. Individual cell voltages are continually monitored and when the voltage of any cell reaches a low selected level approaching zero volts, the individual cells are then shunted by discharge transistors. The battery is recharged by using a charger, generally comprised of an array of solar cells, by disconnecting the load from the charger thereby permitting the entire charger output to recharge the battery. The requirement to monitor both the battery and cell voltage introduces complexity in the reconditioning system. In particular, if a cell transistor should short-circuit, the associated battery cell will be discharged and be incapable of being recharged. Also, if a cell transistor should open-circuit, the associated battery cell can never be fully discharged thereby leaving the battery with unequalized cell voltages and reduced effectiveness. There is also a shunting effect introduced by the cell transistors in their non-conducting and, thus, high-impedance state, which causes the battery cells to slowly discharge through the impedance of the transistors reducing battery load potential. In addition, battery reconditioning is effected in this system of the patent only when the battery or cell voltage drops below a preselected value, rather than at some preselected time in the satellite's mission. During recharging, the battery is connected to the power source and the load is disconnected leaving the load without any source of power. This condition is intolerable for communication satellites which require the communication's equipment to be operable on a 24-hour basis.
There is a need, therefore, to provide a lightweight and reliable reconditioning system which results in the removal or discharge of substantially all the energy stored in each cell without the possibility of cell polarity reversal. In addition, there is a need to provide for equalization of cell voltages to increase battery life and effectiveness. Further, a system is needed to reduce the possibility, if not to eliminate the possibility, of open- or short-circuits occurring in the cell discharge path.